1. Technical Field
This invention relates to methods of manufacturing endovascular prostheses using a deformable matrix.
2. Background Information
The functional vessels of human and animal bodies, such as blood vessels and ducts, occasionally weaken or even rupture. For example, in the aortic artery, the vascular wall can weaken, resulting in dangerous conditions such as aneurysms and dissections. Upon further exposure to hemodynamic forces, such an aneurysm can rupture.
One treatment for aneurysms includes using stent grafts that include one or more stents affixed to a graft material. The stent grafts are placed within vascular networks and are secured at the treatment site by endovascular insertion using inducers and catheters. They are then enlarged radially and remain in place by attachment to the vessel wall. In particular, stent grafts are used to treat descending thoracic and abdominal aortic aneurysms. At one end the stent graft defines a single lumen for placement within the aorta and a distal end is bifurcated to define two lumens extending into the branch arteries. It is important to effectively exclude an aneurysm by providing a seal both proximally and distally to the aneurysm such that a patient's blood flow is shunted through the stent graft and not the diseased vasculature. A device of this type can, for example, treat various arterial aneurysms, including those in the thoracic aorta, abdominal aorta, iliac, or hypogastric artery.
Stent grafts are susceptible to certain latent complications, such as instability leading to kinking, obstruction of the lumen and/or disintegration leading to possible graft explanation. For instance, the stent graft may become displaced from its intended position due to larger displacement forces within the smaller diameter stent graft portions. Stent grafts are also susceptible to different types of endoleaks. In some cases, the endoleaks cause a relapse of the conditions the stent grafts were employed to treat. Endoleaks are sometimes caused or aggravated by graft migration in addition to other factors.
Two closely related aspects of stent graft function, therefore, are sealing and fixation. A stent graft often engages the wall of a lumen on both ends of the aneurysm or other defect at proximal and distal regions referred to as landing or sealing zones. Typically these sealing zones are located near the termini of the stent grafts. The seal between the stent graft and the vascular wall is typically formed at these locations as a result of the circumferential apposition of the stent graft to the vascular wall. This apposition is usually maintained by the radial force exerted by the stents attached to the stent graft.
The term “stent graft” refers to a type of endovascular prosthesis made of a tubular graft material and supported by at least one stent.
The major functional requirements of a covered stent include the usual vasculature support functions typical of a bare stent and the addition of a barrier which may exclude an aneurysm or simply be used to trap plaque against the vascular wall (e.g., as in the case of a carotid stent application). In both cases, it would be desirable for the barrier element to completely cover the perimeter of the stent surface. A secure attachment between the covering and the stent is important because delamination leads to separation of the covering from the stent. This delamination negates the stent graft's effectiveness as a covered stent. Delamination also may lead to “flaps” that may partially occlude the lumen and lead to local turbulent blood flow. This often results in localized or focal stenosis and, possibly, to complete occlusion. Small diameter (e.g., 8 mm) stents are very susceptible to this complication. Larger diameter (e.g., 28 mm) stents are capable of mitigating the localized hemodynamic turbulence.
Complicating the issue is the relatively small diameter (e.g., 8 mm) as compared to endovascular stent-grafts where their large diameters (e.g., 28 mm) mitigate the localized hemodynamic turbulence.
The stent and the graft material of endovascular prostheses are often attached using hand-sewn sutures. This method of attachment is highly skilled, labor-intensive, time-consuming, and expensive.
Another method of attaching the stent and the graft material of an endoluminal prosthesis together is to cover the stent with an adhesive or a polymer coating that will allow the stent to be bonded to the graft material. Unfortunately, this type of attachment has several drawbacks. For example, this technique often requires multiple steps, since the stent must be treated with the adhesive or polymer coating before the process of attaching the graft can begin. Furthermore, the process of coating the stent with the adhesive or polymer coating usually requires multiple steps. Typically, the adhesive or coating is applied in a first step, using a variety of methods, and then must be cured in a subsequent step. In addition, once the adhesive or polymer coating has been applied to the stent and the graft material has been placed over or within the coated stent, actual bonding between the graft material and the adhesive or the polymer coating usually requires heating the coated stent and the graft material in an oven or other heating device. Unfortunately, this heating process limits the types of graft materials that can be used and may also affect the integrity of the graft material itself. In addition, this heating process can also thermoplastically fuse large portions of the graft material.
Current manufacturing methods for polymer covered stents use rigid mandrels made of glass or stainless steel. Several layers of polymer are first added onto a glass or stainless steel mandrel. Then a stent is placed over the polymer and mandrel before several more layers of polymer on added to the albumen.
When polymers are used as coatings in methods using rigid mandrels at times, the polymer layer does not maintain an even coat over the stent graft at times. As a result, the layer may not completely encapsulate the stent graft. If a rigid mandrel is used, intimate contact between the stent outer surface and the covering polymer requires precise sizing. If the stent is too small, it may stick on the mandrel and destroy the covering during the process of extracting it from the mandrel. If it is too large, the covering may simply not encapsulate the stent.